Various types of machinery are extremely dangerous to operate unless closely monitored and controlled by a human operator If the operator's attention is diverted momentarily, or if the operator is injured, it is an essential feature of many types of machinery that a "deadman" switch immediately discontinues operation of the machinery.
For example forging machinery, cranes, metal stamping presses, bulldozers, etc. are recognized as being extremely dangerous unless closely monitored by an attentive operator. Often a deadman switch is a simple spring loaded throttle or hydraulic lever which when released immediately returns to a neutral or brake position. Electric switches are often used as well. For example in punch presses where the operator's two hands must be positioned away from dangerous machinery on two operating buttons simultaneously or the operation of the punch press will immediately be arrested by electric switching means.
Safe design of motorized childrens toy vehicles also include deadman switches. Battery operated vehicles in which children are seated often include a spring loaded imitation throttle which returns to a brake position when released.
A distinct disadvantage of a spring loaded lever type deadman switch is experienced if an operator falls or leans upon the switch. Such a condition may occur during a heart attack for example or, in the case of childrens vehicles, a confused and frightened child may not appreciate that they are leaning against the throttle or that it should be released. Spring loaded deadman switch levers also depend entirely upon springs and other mechanical means to ensure the safety of operation. Such switches may be damaged or tampered with thereby posing significant risk to the operator and those in the immediate vicinity.
Use of electrically operated deadman switches suffer from the disadvantages of all electrical devices in that they are often delicate, must be protected from water and explosion hazards, and control wires are often exposed to damage or corrosion in various applications. In respect of childrens toys often run by batteries, electric switches and controls are notorious for their propensity to be easily rendered inoperable by the simple playful activities and curiosity of innocent children. In industrial applications the expense of constructing a robust waterproof and damage resistant electrical components may be justifiable, however the extremely competitive childrens toy market often results in electrical components having a disappointing short service life.
In many mechanical applications torque transmission devices include clutches, fly wheels, brakes and control means as separate components resulting in a complex assembly. In mass produced items such as childrens toys for example the labour component in assembling such complex transmissions becomes significant.
The ultimate purchaser is often required to keep a stock of spare parts for maintenance and must have a relatively sophisticated capability to maintain such equipment. The complexity of transmission devices is especially disadvantageous when applied to low cost mechanical equipment or toys, or where such equipment will be used in remote areas.
Simplicity of design and ease of maintenance are essential in certain circumstances when the operator is required to maintain the equipment in locations where access to spare parts is limited or costs are prohibitive. It is therefore desirable to construct a transmission device having a deadman switch capability and being relatively simple to maintain and construct.
In many applications in addition to having a deadman capability it may be desirable to design control means such that the operator perceives a direct connection between their movement and the movement of the machinery. For example in a pedal powered childrens toy it may be desirable to include power assist means. The child intuitively appreciates the control function of the pedals in proceeding forward, rearward and stopping.
Motorized wheelchairs and other vehicles for use by the handicapped often do not recognize the psychological benefits of the perception by the operator that a motorized device is operated in a manner substantially the same as a manually driven wheelchair, for example, without the requirement for a high level of physical exertion. In the case of wheelchairs for example, a handicapped person is currently forced to choose between a conventional manually powered wheelchair and a completely motor driven version. If a handicapped person does not possess the required upper body strength to manipulate a manual wheelchair the psychological effect of forcing reliance on a motorized means of transport may be significant.
The operators of industrial and construction machinery may fail to maintain an adequately safe level of attentiveness due to the repetitive nature of their work. It is desirable therefore that means be provided to ensure that operators pay attention to their activities at all times and that machinery controls include a deadman capability.